The present invention relates generally to a ridged reflector for use in optical displays and an optical display device incorporating the reflector. More particularly, the present invention relates to a ridged reflector having optically transmissive properties for use in back-lighted and reflective liquid crystal displays.
Reflectors are often used in optical displays, such as liquid crystal displays, to permit viewing of the displays in ambient light alone. Prior art reflectors include planar specular reflectors and planar diffuse reflectors. Specular reflectors include a substantially planar surface that is covered with a reflective metallic coating. Specular reflectors are characterized by an angle of incidence being substantially equal to the angle of reflection. Diffuse reflectors typically have a roughened surface which is predominately coated with a metallic reflective coating. Diffuse reflectors are characterized by reflecting and scattering incident light. However, neither prior art specular reflectors, nor diffuse reflectors adequately compensate for the effects of glare in optical displays.
Glare represents an unwanted reflection of incident light off any refractive interface associated with a display device. In practice, the refractive interfaces are generally planar with smooth surfaces that are substantially parallel to one another so that the glare from multiple refractive interfaces may be additive. In general, as the difference between refractive indexes increases at the refractive interface, the amount of reflection also increases from the impedance mismatch at the refractive interface. Glare is characterized in that angle of incidence approximately equals the magnitude of the angle of reflection. Glare typically occurs at both glancing incident angles and nonglancing incident angles relative to any refractive interface above the liquid crystal material of the display device. Perceived glare is glare which is coincident with or lies within a preferential viewing cone of an optical display. Perceived glare may be perceived by a viewer and may detract from the usable brightness and the legibility of the display. Actual glare may exist regardless of whether or not, it is actually perceived by a viewer.
Glare may be categorized as primary glare and secondary glare. Primary glare occurs as ambient light is reflected from an exterior face of an optical display. Primary glare is typically more prevalent and bothersome to a viewer than secondary glare. Secondary glare occurs as ambient light is reflected from other refractive interfaces within the display without first reaching the reflector. For example, in a twisted nematic display secondary glare occurs when light entering the display is reflected from indium-tin oxide electrodes.
Commercially available glare-reducing films have been used in optical displays to match different impedances at the refractive interfaces so as to reduce glare reflections. The glare-reducing film generally has a thicknesses which is an integer multiple of a quarter wavelength within the visible light frequency range. However, glare reducing films tend to increase manufacturing costs in a manner which discourages their wide-spread commercial use.
Specular and diffusive reflectors may be further characterized as single-mode or dual-mode reflectors. Single-mode reflectors merely reflect light. Dual-mode reflectors have both a reflective mode and a transmissive mode. Dual-mode reflectors are sometimes referred to as transflectors. The reflective operational mode is desired when using the device in ambient light. The transmissive mode is desired when using the device in the dark or when inadequate ambient light is present. A severe limitation of dual-mode reflectors is that the percentage of transmissiveness of the transmissive mode usually may only be increased at the expense of decreasing the percentage of reflectivity of the reflective mode, and vice versa. For example, typical commercially available dual-mode reflectors may offer 70 percent reflectivity and 30 percent transmissiveness, or 60 percent reflectivity and 40 percent transmissiveness.
Thus, a need exists for a reflector which reduces perceived glare in display devices. In addition, a need exists for a dual-mode reflector which reduces perceived glare, while permitting efficient use and improved legibility of displays with back-lighting, ambient light, or both.